minpoly: add support for algebraic extensions of QQ
minpoly: update documentation

also rename _minpoly_op_algebraic_number to _minpoly_op_algebraic_element
and update error messages
minpoly: add more tests
minpoly: fix doctest error
Improve subs for Derivative.

Perform substitutions if derivative variables are in a different, but
equivalent order and perform substitutions for subderivatives.
Added slow tests to the travis system.
Timeout the slow tests in Travis to less than 10 minutes
Seconds, not minutes
Run the slow tests in Travis with a timeout of 5 minutes
Fix the failing finite_rv slow tests
Add additional tests for Derivative subs method.
polys: implement modular GCD algorithm for univariate integer polynomials
polys: add tests for modgcd_univariate()
polys: implement modular GCD algorithm for bivariate integer polynomials
polys: add tests for modgcd_bivariate()
polys: implement modular GCD algorithm for multivariate integer polynomials
polys: add tests for modgcd_bivariate()
Merge branch 'master' into modgcd

Conflicts:
	sympy/polys/polyerrors.py
polys: change docstirings to be 'raw' strings in modulargcd
polys: rewrite _swap function in modulargcd

No more conversions to dense representation are used for this function.